1. Field of the Invention
The present invention relates to a display control apparatus and, more particularly, to a display control apparatus for a display device having a display element which uses, e.g., a ferroelectric liquid crystal as an operating medium for updating a display state and can hold an updated display state upon application or the like of an electric field.
2. Related Background Art
A display device used as an information display means for achieving a visual information representing function is used in an information processing system or the like. A CRT display device (to be referred to as a CRT hereinafter) is generally used as such a display device.
Various information processing systems such as so-called personal computers are available in accordance with hardware, software, and signal transmission schemes. In this case, CRT display control apparatuses (CRTC) unique to various systems are used. Such CRTCs are exemplified by a VGA81 (available from IBM) as a VGA (Video Graphics Array) dedicated for an information processing system PC-AT (available from IBM) and an 86C911 (available from S3) as an SVGA (Super VGA) obtained such that an accelerator function for displaying predetermined images such as a circle and a rectangle is added to the VGA.
FIG. 1 is a block diagram showing an SVGA arrangement used in a CRTC.
When the host CPU of an information processing system partially rewrites a display memory window area in a host memory space, the rewritten display data is transferred to a VRAM 3 through a system bus 40 and a SVGA 1. The SVGA 1 generates a VRAM address on the basis of the address of the display memory window area and rewrites the display data in the VRAM 3 which is located at this VRAM address.
Meanwhile, the SVGA 1 accesses the VRAM 3 at the same period as the scan period of the CRT and sequentially reads out display data developed in the VRAM 3. The readout data are transferred to a RAMDAC 2. The RAMDAC 2 sequentially converts the input display data into R, G, and B analog signals and transfers the converted analog signals to a CRT 4. The SVGA used as the CRT display control apparatus functions to unconditionally transfer the display data at a predetermined period to the CRT.
In the above CRT display control, since the VRAM 3 comprises a dual port RAM, the VRAM 3 can independently perform an operation of writing display data in the VRAM to update the display information and an operation of reading out the display data from the VRAM. For this reason, the host CPU need not consider display timings and the like at all. Desired display data can be advantageously written at an arbitrary timing.
A CRT requires particularly a length in the direction of thickness of the display screen and has a large volume. It is difficult to obtain a compact CRT as a display device as a whole. This limits the degree of freedom of an information processing system using a CRT as a display. That is, the degrees of freedom in installation locations and portability are decreased.
A liquid crystal display (to be referred to as an LCD hereinafter) can be used as a display device which can compensate for the above drawbacks. More specifically, an LCD can achieve compactness (particularly, a low-profile configuration) of the display device as a whole. Of such LCDs, a display using a liquid crystal cell containing a ferroelectric liquid crystal (to be referred to as an FLC) is available. This display will be referred to as an FLCD hereinafter. One of the characteristic features of the FLCD lies in that the display state of the liquid crystal cell is memorized upon application of an electric field. That is, its liquid crystal cell is sufficiently thin, the elongated FLC molecules in the cell are aligned in the first or second stable states in accordance with an electric field application direction, and the aligned state of the molecules is maintained after the electric field is withdrawn. The FLCD has a memory function due to the above bistable operations of the FLC molecules. The details of the FLC and FLCD are described in U.S. Pat. No. 4,964,699.
Although the FLCD has the above memory function, it has a low FLC display updating speed. The FLCD cannot follow up with changes in display information which must be instantaneously updated. Such operations are exemplified by cursor movement, a character input, and scrolling.
In FLCDs having the above characteristics, various display drive modes which have originated from these characteristics or compensate for these characteristics are available. More specifically, in refresh driving for sequentially and continuously driving scan lines on the display screen as in a CRT and any other liquid crystal display, a relatively large time margin is available in its drive period. In addition to this refresh driving, partial rewrite driving for updating the display state of a part (line) subjected to a change on the display screen and interlace driving for interlacing and driving scan lines on the display screen are also proposed. The display information change speed can be increased by the partial rewrite driving or the interlace driving.
If display control of the FLCD having the above advantages can be performed using an existing CRT display controller, an information processing system using an FLCD as a display device can be arranged at a relatively low cost.
It is difficult to arrange an FLCD having continuous multi-gradation in display color tones as compared with the CRT. As a means for overcoming this difficulty, a binary process is performed in a display having a smaller number of colors in accordance with an error diffusion method, an ED method, or a dither method. Therefore, apparent multi-gradation display is performed.
A hardware cursor is a function of smoothly displaying, on the display screen, a cursor moved on the display screen at high speed, in such a manner that cursor position information and cursor shape information are provided in addition to image information present in the VRAM and are output to the display device using a superimposition function.
In the prior art, however, when the binary process is performed on the display side, information representing whether an object can be processed by the binary process is received from a display control unit in the form of area separation information or is determined on the display side in accordance with the image data contents. In either method, in a sprite representation called a mouse display, when the binary process is performed without area separation, the edge of the sprite is not emphasized to make it difficult to visually recognize the sprite because the sprite is moved on the display screen at high speed. In addition, when the sprite is moved on the display screen, the binary operation of its portion and neighboring pixels may result in an effect image different from the expected one. This degrades the image quality. A pattern designated or instructed and displayed by the sprite function receives the most attention by the user on the display screen. Even slight degradation within a small range cannot be neglected.
The function of supporting the hardware cursor also has the following drawbacks.
(1) When a hardware cursor is moved at high speed, the image of the cursor is distorted in accordance with a partial rewrite sequence.
(2) When a hardware cursor is moved at high speed, a high-speed partial rewrite operation must be performed.
When a partial rewrite operation is performed in a mouse cursor preferential display mode, and the screen rewrite speed is decreased, the display quality of objects moved in synchronism with the mouse cursor is undesirably degraded.
FIG. 2 shows a case in which a window is moved in synchronism with a mouse. This case exemplifies a window system such as Microsoft Windows (available from Microsoft). The user moves the mouse cursor to the title bar at the upper portion of the window and drags the cursor to move the window. In this case, the window and the cursor are synchronously moved. When the mouse display has a preference over the window display, the display quality of the moving window is degraded.
An FLCD partial rewrite operation is performed for each horizontal line. When the mouse cursor display has a preference over the line display, the drawing timing of the line on which the mouse cursor is located is shifted from any other line, resulting in poor display.